1. Field of the Invention
The present invention relates to a magneto-optical recording medium suitable for recording information thereon on a magnetic modulation recording system.
2. Description of the Prior Art
A magneto-optical recording method heats a magnetic thin film locally at a temperature beyond the Curie point or the compensation temperature to extinguish the coercive force of the heated portion of the magnetic thin film, and applies an external recording magnetic field to the heated portion to invert the direction of magnetization of the heated portion in the direction of the recording magnetic field. The magneto-optical recording method has been practically applied to external storages for optical filing systems and computers, sound recording apparatuses and video recording apparatuses.
A known magneto-optical recording medium for use in carrying out the magneto-optical recording method consists of a transparent base plate formed of polycarbonate or the like, and a laminate recording layer formed on one major surface of the transparent base plate by sequentially superposing a recording magnetic layer having an easy direction of magnetization perpendicular to the surface thereof and a high magneto-optical effect, such as a thin film of an amorphous alloy of a rare earth metal and a transition metal, a reflecting layer and a dielectric layer. A laser beam is projected on the magneto-optical recording medium from the side of the other major surface of the transparent base plate when reading signals recorded on the magneto-optical recording medium.
Magneto-optical recording methods are classified roughly into optical modulation recording methods and magnetic modulation recording methods. Magnetic modulation recording methods are expected to be potential magneto-optical recording methods owing to their overwriting capability.
The magnetic modulation recording method writes information on a magnetic thin film by inverting a magnetic field applied to the magnetic thin film at a high frequency. The magnetic modulation recording method, in general, uses a magnetic head provided with a magnetic field creating means capable of magnetic field inversion, such as a coil or the like. A magnetic head capable of applying a high-frequency inverting magnetic field to the magnetic thin film is able to create only a magnetic field of a very low intensity on the order of 100 Oe due to various restrictions, and the magnitude of the magnetic field decreases with recording frequency due to increase in the temperature of the coil. Accordingly, a magneto-optical recording medium to be used by the magnetic modulation recording method is required to be capable of stably recording signals even if the intensity of the magnetic field is not higher than 100 Oe. However, the sensitivity of the conventional magneto-optical recording medium is not high enough to record signals by using a low-intensity magnetic field having a field intensity on the order of 100 Oe, and even a slight variation in the distance between the magnetic head and the magneto-optical recording medium will cause a drop-out of recording signals. Particularly, when the magneto-optical recording medium is used on a recording/reading apparatus liable to be exposed to vibrations and shocks, such as an automotive audio apparatus or the like, the practical characteristics of the magneto-optical recording medium is unsatisfactory. Thus, the magneto-optical recording medium is unsuitable for the magnetic modulation recording method.